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Polynuclear alkoxy–zinc complexes of bowl-shaped macrocyclesand their use in the copolymerisation of cyclohexene oxide andCO2

Citation for published version:Pankhurst, JR, Paul, S, Zhu, Y, Williams, CK & Love, JB 2019, 'Polynuclear alkoxy–zinc complexes of bowl-shaped macrocycles and their use in the copolymerisation of cyclohexene oxide and CO2', DaltonTransactions. https://doi.org/10.1039/C9DT00595A

Digital Object Identifier (DOI):10.1039/C9DT00595A

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Received 00th January 2017,

Accepted 00th January 2017

DOI: 10.1039/x0xx00000x

www.rsc.org/

Polynuclear alkoxy-zinc complexes of bowl-shaped macrocycles and their use in the copolymerisation of cyclohexene oxide and CO2

James R. Pankhurst,a Shyeni Paul,b Yunqing Zhu,b Charlotte K. Williams,b* and Jason B. Lovea*

The reactions between alcohols and the tetranuclear ethyl-Zn complexes of an ortho-phenylene-bridged polypyrrole

macrocycle, Zn4Et4(L3) 1 and the related anthracenyl-bridged macrocyclic complex, Zn4Et4(THF)4(L4) 2 have been studied.

With long-chain alcohols such as n-hexanol, the clean formation of the tetranuclear hexoxide complex Zn4(OC6H13)4(L3) 3

occurs. In contrast, the use of shorter-chain alcohols such as i-propanol results in the trinuclear complex Zn3(μ2-OiPr)2(μ3-

OiPr)(HL3) 4 that arises from demetalation; this complex was characterised by X-ray crystallography. The clean formation of

these polynuclear zinc clusters allowed a study of their use as catalysts in the ring-opening copolymerisation (ROCOP)

reaction between cyclohexene oxide and CO2. In-situ reactions involving the pre-catalyst 1 and n-hexanol formed the desired

polymer with the best selectivity for polycarbonate (90 %) at 30 atm CO2, whilst the activity and performance of pre-catalyst

2 was poor in comparison.

Introduction

Multidentate macrocycles are attractive as ligands for di-

and polynuclear complexes of transition- and f-block metals as

they can control both the basic coordination chemistry and the

relative spatial positioning of metals within the macrocyclic

framework, so providing a pre-organised chemical

environment.1-4 This ligand design strategy can deliver a

diversity of physical and reaction properties in the resulting

complexes leading to, for example, clustering and aggregation,5-

14 catalytic activity,15-28 molecular magnetism,29 allosteric

constructs,30, 31 and molecular sensing.32-35

We have been studying macrocycles in which two donor

compartments comprising two dipyrromethane and two Schiff-

base nitrogen donors (i.e. an N4-donor set) are separated by

rigid aryl backbones (e.g. L3 and L4, Figure 1).36, 37 On metalation,

the resulting dinuclear complexes adopt Pac-Man structures

(e.g. A, Figure 1) that promote a diversity of chemistry within

the dinuclear molecular cleft, including dioxygen reduction

catalysis,38-41 halide sensing,42 and uranyl reduction and oxo-

group functionalisation.43-50 We have also exploited a steric

variation of the meso-substituent (H instead of alkyl, L1 and L2,

Figure 1) which results in the adoption of bowl-shaped

structures on metalation, hinging at the meso-carbon instead of

the aryl groups.51 Importantly, using this latter ligand variant

allows for the isolation of higher nuclearity complexes such as

the tetranuclear zinc alkyl macrocyclic complexes 1 and 2

(Scheme 1); these complexes undergo subsequent protonolysis

reactions with water to form tetranuclear Zn-oxo and hydroxo

clusters.52

Figure 1. Schiff-base pyrrole macrocycles with varying meso-substituents and aryl linkers

and the formation of generic dinuclear complexes of Pac-Man structures.

The straightforward syntheses of 1 and 2, and their facile

hydrolysis, provides an opportunity to study the ring opening

copolymerisation (ROCOP) of carbon dioxide and epoxides to

produce aliphatic polycarbonates.53-57 ROCOP catalysts are

often isolated Lewis-acidic metal-alkoxide complexes or are

pre-catalysts that are activated by alcohols to form metal

alkoxides in situ. 58-75 Furthermore, homogeneous zinc catalysts

a. EaStCHEM School of Chemistry, The University of Edinburgh, Joseph Black Building, David Brewster Road, Edinburgh, EH9 3FJ, UK. Email: jason.love@ed.ac.uk.

b. Chemistry Research Laboratory, 12 Mansfield Road, University of Oxford, Oxford, OX1 3TA, UK. Email: charlotte.williams@chem.ox.ac.uk.

Electronic Supplementary Information (ESI) available: Synthetic details and characterising data, detailed catalysis results. See DOI: 10.1039/x0xx00000x

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are attractive for ROCOP as the metal is redox inert and

sustainable. Zinc clusters formed by alcoholysis/hydrolysis of

organo-zinc species act as ROCOP catalysts but have very slow

rates.76 Highly active zinc -diketiminate (BDI) catalysts were

reported,77 with the best forming dimers under the

polymerisation conditions.78-84 Dinuclear zinc macrocyclic

complexes are also highly active and operate under low

pressures of carbon dioxide.25, 84-88 While higher nuclearity zinc

catalysts have been reported, it is not yet understood if

dinuclear catalysts are optimum.89-93 As such, we reasoned that

the tetranuclear alkyl-zinc macrocyclic complexes 1 and 2 could

be activated by alcoholysis and that the resulting zinc alkoxide

complexes could act as catalysts for ROCOP of CO2 and

epoxides.

Results and discussion

Multinuclear ZnII complexes considered for ROCOP catalysis

The two tetranuclear Zn-alkyl complexes [Zn4Et4(L)], where L is

either the ortho-phenylene-bridged macrocycle L1 (1) or the

anthracenyl-bridged macrocycle L2 (2), were prepared as

previously described (Scheme 1).52 These complexes are inert

towards insertion of CO2, but undergo protonolysis reactions

with four equivalents of n-hexanol to generate Zn-alkoxide

complexes. Specifically, the alkoxide complex, [Zn4(μ2-

OC6H13)4(L1)] (3) was isolated, in 84 % yield, from the reaction of

1 with four equivalents of n-hexanol in THF (Scheme 1). The

reaction proceeds immediately as evident from the ethane gas

evolution observed. The 1H NMR spectrum of 3 implies that it is

fully symmetric with a single set of resonances for the

macrocycle that are shifted in comparison with 1 (Figure S1); in

C6D6, the imine protons appear as a single resonance at 8.07

ppm, and the meso-protons appear at 6.38 ppm. Importantly,

the ethyl resonances, that appear at 1.32 and 0.42 ppm for 1 in

C6D6, are absent from the spectrum of 3. Instead, there are a

number of overlapping resonances between 1.89 and 0.57 ppm

assigned to the new hexyl alkoxide ligands. Two triplet

resonances, at 3.83 and 3.70 ppm, each showing integral values

consistent with four protons are assigned to the methylene

groups adjacent to the Zn-O bond. The distinct chemical shifts

indicate that the alkoxide ligands bridge between two metals,

with two alkoxides bridging between imine-donors and the

other two bridging between pyrrole donors; the structurally

characterised and analogous Zn-hydroxide complex, [Zn4(μ2-

OH)4(L1)], also displayed similarly equivalent macrocycle

resonances yet two distinct hydroxide environments.52

Furthermore, the 19F NMR resonance for the ortho-F groups is

severely broadened due to restricted rotation of that group;

such broadening is typically observed for bowl-shaped

tetranuclear complexes.51 As such, the NMR data support the

protonolysis of 1 to form 3 which is a bowl-shaped, tetranuclear

Zn-(μ2-alkoxide) complex.

Scheme 1. Tetranuclear ethyl-zinc complexes of the Schiff-base pyrrole macrocycles L3

and L4 and their reactions with alcohols; complexes 1 and 2 were reported previously.52

Protonolysis reactions between 1 and alcohols other than n-

hexanol are not straightforward. The reaction of 1 with iso-

propanol occurs readily, evolving gas from the THF solution, to

yield the new trinuclear complex, [Zn3(μ2-OiPr)2(μ3-OiPr)(HL1)]

(4, Scheme 1). The 1H NMR spectrum of 4, in d8-THF at 300 K

shows a number of broad resonances consistent with the

formation of a symmetric product and with the successful loss

of the ethyl groups from 1 (Figure S3). The broad NMR

resonances suggests the complex has a fluxional solution

structure and so a VT-NMR study was undertaken. At 213 K, the

spectrum is sharper and consistent with an asymmetric

macrocyclic ligand environment, with each of the four

inequivalent imine proton resonances showing signals at 8.86,

8.68, 8.48 and 8.40 ppm (Figure S4). Notably, a resonance at

11.98 ppm is only observable at this temperature and is

assigned to a single pyrrole N-H proton. In addition, only three

iso-propoxyl ligands are seen, with the ipso-protons appearing

as a single, broad resonance centred at 4.29 ppm, and the six

individual methyl groups well resolved between 1.45 and 0.66

ppm. At 330 K, broad, thermally averaged resonances are seen,

with the imine protons appearing as a single resonance at 8.36

ppm, whilst the three iso-propoxide ipso-protons resonate at

4.11 ppm; the associated methyl protons, with integral values

of 18 protons per macrocycle, show a signal at 1.02 ppm.

Large, red, block crystals of 4 were grown from a benzene

solution and the solid-state structure was determined by X-ray

crystallography. Complex 4 is a trinuclear complex (Figure 2)

and adopts a highly distorted bowl-structure with a bite-angle

of 102° between the two N4-donor compartments of the

macrocycle. This bite-angle is small in comparison with other

bowl-shaped complexes of the same ligand, for example, its CuII

analogue (Cu2(py)4(L1), 152°).51 This small bite-angle is

attributed to coordination of the ligand to an L-shaped,

trinuclear Zn-iso-propoxide cluster, which resembles a cubane

in which two vertices are removed 94-96. In this cluster, the Zn

centres are bridged by two μ2-alkoxide ligands (O2 and O3) and

one μ3-alkoxide ligand (O1). Each Zn centre is four-coordinate

with highly distorted tetrahedral coordination geometries, with

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bond angles ranging from 81.58(8)o to 143.47(9)o, and in order

to accommodate this, an imine-group (N5) from one of the

imino-pyrrole chelates is non-coordinating. The inter-metallic

distances between nearest neighbours in the cluster are

3.0071(5) Å (Zn1-Zn3) and 2.8213(6) Å (Zn2-Zn3). The Zn-O bond

lengths that describe the edges of the cluster are regular and

are in the range 1.920(2) Å to 2.143(2) Å. However, the cluster

is distorted, with inequivalent bond angles in the hinge, of

121.4(1)o (Zn2-O1-Zn1) and 111.21(8)o (O3-Zn3-O2).

The reaction between 1 and four equivalents of phenol

occurs readily and results in the formation of a complex that

displays a similar 1H NMR spectrum to that of 4 (Figure S5). A

single N-H proton resonance is seen at 11.76 ppm (at 300 K)

which indicates that a similar demetalation reaction has

occurred to form [Zn3(OPh)3(HL1)]. In an attempt to introduce a

kinetic barrier towards demetalation, the reaction between 1

and 2,6-di-tert-butyl-phenol was investigated. No reaction is

seen at room temperature, with the 1H NMR spectrum of 1

unchanged in the presence of the di-substituted phenol.

However, after heating at 90 oC for 24 h, the 1H NMR spectrum

shows that while partial protonolysis had occurred, no N-H

proton is seen, consistent with the reaction avoiding

demetalation side-processes (Figure S6). Nonetheless, the

triplet resonance at 6.91 ppm, assigned to the para-proton of

phenoxide co-ligands, shows an integral value consistent with

there being only two phenoxides per macrocycle. There is also

a quartet at 0.55 ppm and its integral is consistent with there

being two ethyl ligands per macrocycle. Thus, the product is the

tetranuclear complex [Zn4(OC6H3-tBu2-2,6)2Et2(L1)]. Although

zinc-phenoxide complexes are able to initiate ROCOP,97 this

heteroleptic complex was not investigated further as the

mixture of co-ligands would likely complicate initiation

processes. Overall, the attempted protonolysis reactions

resulted in only 3 as an isolated catalyst suitable for the ROCOP

and only n-hexanol was considered as an acceptable alcohol for

the in situ generation of catalytic systems using 1 and 2.

Figure 2. Solid-state structure of 4 (displacement ellipsoids drawn at 50 % probability).

For clarity, solvent molecules and all hydrogen atoms except the meso- and N-H

hydrogen atoms are omitted. Right: orthogonal views of the Zn3(OiPr)3 cluster.

Demetalation of a tetranuclear Zn-iso-propoxide complex,

that presumably forms initially, would yield one equivalent of

Zn(OiPr)2 per equivalent of 4. The relative instability of the

tetranuclear zinc complex suggests that combining 1 or 2 with

iso-propanol will not be an effective initiating system as the

desired multinuclear zinc alkoxide complex will be

contaminated by the homoleptic zinc alkoxide. Indeed,

homoleptic zinc alkoxide complexes are known to catalyse the

formation of ether linkages in ROCOP reactions.98 Demetalation

was not observed during the reaction of 1 with n-hexanol, which

may be a result of the longer-chain alkoxide ligands imparting

kinetic stability. The pKa for n-hexanol is predicted at 16.6 in

water99 and is essentially identical to that of iso-propanol (pKa =

16.5 in water).100 Therefore, whilst demetalation occurs

through protonolysis of the Zn-alkoxide bond, the formation of

4 is not attributed to a difference in acidity of the alcohol.

Polymerisation catalysis

Ring-opening copolymerisation reactions were conducted using

complex 1 reacted in situ with four equivalents of n-hexanol,

with a catalyst loading of 0.1 mol%, in neat cyclohexene oxide

(CHO), under 1 bar pressure of CO2, at 80 °C for 24 h (Table 1,

entry 1). Four equivalents of the alcohol (0.4 mol%) were added

immediately before the mixture was exposed to carbon dioxide.

The catalytic activity was low, with a TOF of 9 h-1. The polymer

formed has a low molar mass (Mn = 4400 g/mol) and broad

dispersity (Đ = 1.67). Analysis of the polymer composition using 1H NMR spectroscopy showed that the majority of linkages are

ether, with only 7% carbonate linkages. Complex 1 was also

tested, under 1 bar CO2, using four equivalents of methanol as

the alcohol (Table 1, entry 2). By analogy to the stoichiometric

reactions with iso-propanol, it was proposed that a trinuclear

Zn-methoxide complex would form and this species shows a low

catalytic activity (TOF = 13 h-1). The resulting poly(ether-

carbonate) shows a high proportion of ether linkages, moderate

molar mass (Mn = 15,300 g/mol) and broad dispersity (Đ = 2.69),

the latter indicative of slow or multiple initiation reactions.

In order to increase the proportion of carbonate linkages for

the catalyst system comprising 1/hexyl alcohol, the CO2

pressure was increased (Table 1, entries 3 and 4, Figure S8).

Using 30 bar pressure of CO2, both the activity (TOF = 21 h-1) and

the selectivity for carbonate linkages increased (carbonate

linkages = 56 %). In line with the greater conversion, the

resulting polymer shows a higher molar mass (Mn = 18,100

g/mol) but the dispersity remains very broad (Đ = 3.03). When

the reaction pressure is increased further to 50 bar, the catalyst

activity, conversion of epoxide, and carbonate selectivity all

decrease. This may be a result of gas expansion which is known

to occur under such sub-critical conditions and which effectively

dilutes the catalyst concentrations.101

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Table 1. Polymerisation catalysis results using pre-catalysts 1 and 2, and catalyst 3 in the ROCOP of cyclohexene oxide (CHO) and CO2.

Entry Catalyst [Epoxide] / M

([Cat.] /

mol%)

[CO2] / atm TON(a) TOF(b) / h-1 polycarbonate

linkage

selectivity(c)

/ %

Mn g/mol

(Đ) (d)

1 1 10 (0.1) 1 220 9 7 4400 (1.67)

2* 1 10 (0.1) 1 310 13 6 15300 (2.69)

3 1 10 (0.1) 30 510 21 56 18100 (3.03)

4 1 10 (0.1) 50 140 6 29 13700 (2.97)

5 1 5 (0.2) 30 360 15 90 11900 (14)

6 1 5 (0.2) 1 0 0 0 -

7 2 10 (0.1) 1 70 3 0 -

8 2 10 (0.1) 50 30 1 68 -

9 2 5 (0.2) 30 10 0.5 88 -

10 3 5 (0.2) 30 250 11 81 7700 (13)

Reactions were conducted for 24 h at 80 °C and when using 1 or 2, four equivalents of n-hexanol (except where stated otherwise) were added immediately prior to the

addition of carbon dioxide. Reactions were either conducted in neat epoxide (i.e. [CHO] = 10 M) or in toluene ([CHO] = 5 M).* Methanol was added instead of n-

hexanol. (a) TON = (moles epoxide consumed)/(moles catalyst), the conversion was determined by integration of the signals, in the 1H NMR spectrum for methine

protons assigned to CHO (3.14 ppm) and polymer (4.65 ppm). (b) TOF = TON/time (h) (c) Selectivity for carbonate linkages was determined by comparison of the

relative integrals in the 1H NMR spectrum for the signals of polycarbonate (4.65 ppm) and ether linkages (3.43 ppm). (d) The molar mass (Mn) and dispersity (Đ) values

were determined using size-exclusion chromatography (SEC), in THF, which was calibrated with polystyrene standards.

As part of attempts to improve the polymerisation

selectivity, polymerisations were conducted in toluene

solutions to reduce the overall epoxide concentration and

hence slow sequential enchainment reactions (Table 1, entry 5,

6, Figure S9). Overall, the absolute catalyst concentration was

the same as in the previous reactions conducted in neat epoxide

but its relative loading compared to epoxide is increased.

Polymerisations conducted in toluene solution at 1 bar CO2

pressure were unsuccessful (Table 1, entry 6), but at 30 bar

pressure polymerisation occurs to form a polymer with

significantly increased carbonate linkages (Table 1, entry 5).

However, the ROCOP activity is reduced compared to reactions

in neat epoxide, for example the TOF decreased from 21 h-1 (10

M) to 15 h-1 (5 M) (Table 1, entries 3 and 5). The polymerisation

control is very poor forming a polymer with an exceptionally

broad dispersity (Mn = 11,000 g/mol; Ð = 14). To investigate

further, the evolution of polycarbonate molar mass vs.

conversion was analysed (Table S1, Figure S11). At low

conversions, bimodal molar mass distributions are seen

showing a characteristic very high molar mass peak (Mn = 194,

000 g/mol; Ð = 1.89) and a lower molar mass peak (Mn = 2400;

Ð = 3.00). The higher molar mass peak did not increase

particularly as polymerisation progressed whereas the lower

peak shows a clear increase in molar mass vs. conversion.

Aliquots were taken and the 1H NMR spectra shows the

formation of both carbonate and ether linkages throughout the

reaction. It is tentatively proposed that the higher molar mass

peak is due to uncontrolled and rapid formation of polyether,

whilst the lower molar mass peak arises from ROCOP to form

predominantly polycarbonate. Nonetheless, a more detailed

analysis is precluded by the very broad molar mass distributions

that clearly signal problems with relative initiation rates and

number of active sites.

Polymerisations were also conducted under a range of

similar conditions using the catalyst system formed from 2

(Table 1, entries 7-9). Under all conditions, its activity is very

low, although the carbonate selectivity could be somewhat

increased at higher pressures. The isolated hexyl alkoxide

complex 3 shows similar performance to the catalyst system

formed using 1 and n-hexanol, and is consistent with 3 being the

true initiating species formed during alcoholysis of 1. The

polycarbonate product, formed using 3, shows a similar

molecular weight (Mn = 7400 g/mol) and very broad dispersity

(Ð > 13) to that formed using the catalyst system of 1/hexyl

alcohol. Finally, ROCOP reactions using propylene oxide and

carbon dioxide (50 bar) was unsuccessful with all catalysts.

Overall, the activity values for all catalysts are at the lower end

in this field and cannot compete with leading catalysts, such as

the di-zinc catalysts coordinated by -diketiminate or

macrocyclic ancillary ligands.77-88

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Conclusions

The result of reactions between the tetranuclear ethyl zinc

complex 1 and alcohols is highly dependent on the alcohol used.

While reaction with n-hexanol provides the isolable

tetranuclear Zn hexyl-alkoxide complex 3, use of isopropanol

results in demetalation and the formation of the trinuclear Zn

complex 4. Reactions between 1 and phenol similarly result in

demetalation, while the use of the more sterically hindered

alcohol HOC6H3-tBu-2,6 maintains the nuclearity of the complex

but limits the protonolysis reaction, with two ethyl groups

untouched. Complex 1 showed some activity and selectivity as

a catalyst in ring-opening copolymerisation of cyclohexene

oxide and carbon dioxide, with optimised conditions of 30 atm

pressure of CO2, 0.1 mol% catalyst loading, 80 °C and in 5 M

cyclohexene oxide (diluted in toluene). These conditions

enabled the production of polycarbonates with 90% selectivity

for carbonate linkages and with a TOF of 15 h-1. However, the

polymers produced have very broad molar mass distributions

suggesting that multiple catalytic sites are present which exhibit

poor reaction control. The analogous anthracenyl-bridged

complex 2 showed even lower activity and a similar lack of

polymerisation control. While higher nuclearity macrocyclic

zinc complexes have potential as catalysts in ROCOP reactions,

the complexes used in this study appear too labile, with facile

demetalation occurring under reaction conditions, making

them unsuitable as catalysts. This highlights the need for

improved ligand design and complex stability towards alcohols

to prepare more active and selective catalysts for ROCOP

reactions.

Acknowledgements

We thank the University of Edinburgh, the Principal’s Career

Development Scholarship Scheme for funding (JRP), the EPSRC

(EP/L017393/1; EP/K014668/1) and the Chinese Scholarship

Council (studentship to YZ) for their support.

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